5HT2A receptor gene and bipolar affective disorder

Serotonin 5-HT2A receptor activity mediates adipocyte differentiation through control of adipogenic gene expression

Serotonin 5-HT2 receptors are expressed in many tissues and play important roles in biological processes. Although the 5-HT2A receptor is primarily known for its role in central nervous system, it is also expressed in peripheral tissues. We have found that 5-HT2A receptor antagonists inhibit human subcutaneous primary adipocyte differentiation. We also show that siRNA knockdown of the 5-HT2A receptor blocks differentiation. Using gene expression analysis in combination with receptor antagonists we found that activity of 5-HT2A receptors is necessary very early in the differentiation process to mediate expression of adipogenic genes, including peroxisome proliferator-activated receptor gamma (ppar-γ), adipocyte protein 2 (aP2), adiponectin, and serine/threonine-protein kinase 1 (sgk1). We show here for the first time that 5-HT2A receptor activity is necessary for differentiation of human primary subcutaneous preadipocytes to adipocytes, and that 5-HT2A receptor activity mediates key genes related to adipogenesis during this process. Importantly, this work contributes to a greater understanding of the adipocyte differentiation process, as well as to the role of 5-HT2A receptors in peripheral tissues, and may be relevant to the development of novel therapeutic strategies targeting this receptor for the treatment of obesity related diseases.

The ALDH2 and 5-HT2A genes interacted in bipolar-I but not bipolar-II disorder

OBJECTIVE

Clarifying the similarities and differences between the two most common subtypes of bipolar disorder, bipolar-I and bipolar-II, is essential for improving our understanding of them. Because the serotonergic system has been implicated in the pathogenesis of bipolar disorder, it may be important to investigate genes such as the aldehyde dehydrogenase 2 (ALDH2) and serotonin 2A receptor genes, which are involved in metabolizing serotonin and encoding serotonin receptors. We examined the association of the ALDH2 and 5-HT2A-A1438G polymorphisms with bipolar I and II and possible interactions between these genes.

METHODS

One thousand forty-nine participants were recruited: 249 with bipolar-I, 456 with bipolar-II, and 344 healthy controls. The genotypes of the ALDH2 and 5HT2A-A1438G polymorphisms were determined using polymerase chain reactions plus restriction fragment length polymorphism analysis.

RESULTS

Logistic regression analysis showed a significant effect of the ALDH2 and the 5-HT2A-A1438G polymorphisms, and a significant interaction effect for the A/G genotypes of the 5-HT2A-A1438G polymorphism and the ALDH2*1*1 genotypes (p=0.004) discriminated between bipolar-I patients and controls without bipolar disorder. These polymorphisms, however, were not associated with bipolar-II disorder.

LIMITATIONS

The significant differences of age and gender between patients and controls limit the comparison, although statistical adjustments were made for them.

CONCLUSION

Our findings provide initial evidence that the ALDH2 and 5-HT2A genes interact in bipolar-I but not in bipolar-II disorder. Our findings suggest a unique genetic distinction between bipolar-I and bipolar-II.

Excess of transmission of the G allele of the -1438A/G polymorphism of the 5-HT2A receptor gene in patients with schizophrenia responsive to antipsychotics

BACKGROUND

The -1438A/G polymorphism of the 5-HT2A gene has been found to be associated with clinical response to clozapine and other second generation antipsychotics. Testing the impact of this marker on response to first generation antipsychotics (which have a lower affinity for the 5-HT2A receptor) provides the opportunity to help disentangling the two different roles that this polymorphism might have. A psychopharmacogenetic role should be detected only for antipsychotics with high affinity to the 5-HT2A receptor (therefore to second generation antipsychotics). An alternative role would imply tagging a subgroup of patients responsive to any antipsychotic, whatever their affinity, meaning that the association is more depending on non pharmacological charaterictics, such as clinical specificities.

METHODS

A family-based sample of 100 Algerian patients with schizophrenia (according to DSM-IV criteria) and their 200 biological parents was recruited, in order to avoid stratification biases. Patients were all treated, or have been treated, by conventional antipsychotics (mainly haloperidol) for at least four weeks, at appropriate dosage. May and Dencker scale was used to distinguish responders and non responders.

RESULTS

No allele of the -1438A/G polymorphism of the 5-HT2A gene was transmitted in excess (50 transmitted for 38 untransmitted) in the whole sample of patients with schizophrenia (p = .90). In contrast, a significant excess of transmission of the G allele was observed (p = .02) in the subgroup of patients with good treatment response (17 transmitted for 6 untransmitted).

CONCLUSION

Using a TDT approach, we showed that the G allele of the -1438A/G polymorphism of the gene coding for the 5-HT2A receptor was associated to schizophrenia with good response to conventional antipsychotics, although this conclusion is based on 88 informative patients only. Because previous data showed the same result with atypical antipsychotics, it can be concluded that the G allele tags a subgroup of schizophrenic patients with greater chance of improvement with antipsychotics of either type.

Negative symptoms of schizophrenia could explain discrepant data on the association between the 5-HT2A receptor gene and response to antipsychotics

Pharmacogenetic studies assessing the role of 5-HT(2A) receptor gene in antipsychotic efficacy yielded conflicting data. Phenotypical heterogeneity of schizophrenia might explain such discrepancies. For example, negative symptoms are known to reflect severity of illness and to restrain therapeutic response. On this basis, we re-assessed the possible influence of the -1438A/G polymorphism of the 5-HT(2A) receptor gene on the clinical efficacy of atypical antipsychotics with focus on several relevant dimensions. One hundred and sixteen French schizophrenic subjects treated for at least 1 month by atypical antipsychotics were screened for treatment response according to the May and Dencker scale. Gender, age at onset, duration and severity of illness, intensity of negative and positive symptoms at discharge were investigated. The intensity of negative symptoms at discharge was the only variable explaining May and Dencker score (p < 0.001), and was significantly associated with the AA genotype of the -1438A/G polymorphism of the 5-HT(2A) receptor gene (p = 0.03). However, the A allele was not independently associated with refractoriness to atypical antipsychotics. Accordingly, the score reached in the Scale for the Assessment of Negative Symptoms (SANS) appeared as a confounding factor between therapeutic response and the -1438A/G polymorphism of the 5-HT(2A) receptor gene, at least in our sample. This data indicate that negative symptoms are worth being systematically assessed in pharmacogenetic studies aimed at analysing candidate genes in schizophrenia.

Lack of association between 5HT2A receptor gene haplotype, bipolar disorder and its clinical subtypes in a West European sample

Bipolar affective disorder (BPAD) is a complex psychiatric disorder with a major genetic contribution. Abnormalities in serotonergic function have been implicated in its aetiology. The 5HT2A receptor (5HT2AR) gene is a strong candidate gene for involvement in BPAD, but previous association studies have reported conflicting results. These data are difficult to interpret because most negative results were obtained with small samples. The aim of this study was to test the association between the 5HT2AR gene and BPAD in a large West European sample. We studied the -1438G/A and the His452Tyr polymorphisms, for haplotype analysis to increase both informativity and the likelihood of detecting an association between BPAD and the 5HT2AR gene. We analysed the genotype, allele and haplotype distributions of two 5HT2AR gene variants in a population of 356 BPAD patients, which we compared with 208 healthy controls. We also carried out exploratory analysis in clinical subgroups of patients defined according to personal history of mood disorders, suicidal behaviour, comorbid psychiatric disorders and family history of affective disorders. We found no difference between BPAD patients and controls for allele, genotype and haplotype distributions. Exploratory analysis in subgroups of BPAD patients showed only a marginal difference in haplotype distribution between controls and BPAD patients with antidepressant-induced mania (P = 0.018). This difference was not significant after correction for multiple testing. Our study suggests that the 5HT2AR gene is unlikely to be involved in genetic susceptibility to BPAD but should be further investigated in a pharmacogenetic study.

Serotonergic polymorphisms and psychotic disorders in populations from North Spain

There is strong biological evidence relating alterations in the serotonergic system with mental disorders. These alterations may be originated at the DNA level by sequence mutations that alter the functioning of serotonin receptors and transporter. To test this hypothesis we investigated three genetic variants of the 5-HT2A receptor (-1438G/A, 102T/C and His452Tyr) and two variants of the serotonin transporter (a VNTR in the second intron and a 44 bp insertion/delition in the promoter region of the gene) in a clinical sample recruited in a human isolate and in surrounding areas in Northern Spain (N = 257) and in ethnically matched controls (N = 334). No clear association was found between 5-HT2A variants and psychosis. However, marginal associations were observed between the 5-HTT LPR and VNTR variants and psychosis (P < or = 0.05) indicating a minor contribution to psychosis of genetic alterations in this gene.

No association between two polymorphisms at the 5HT2A gene and bipolar affective puerperal psychosis

OBJECTIVE

To examine whether variation at two common polymorphisms, T102C and -1438AG, of the serotonin 2A gene (5HT2A) are involved in the puerperal triggering mechanism of bipolar affective puerperal psychosis.

METHOD

A total of 242 parous women diagnosed with bipolar disorder were genotyped for the two polymorphisms. Of these, 165 women had experienced a manic or psychotic episode, according to DSM-IV criteria, within 6 weeks of childbirth (the puerperal psychosis group). The comparison group comprised of 77 parous women who had not experienced psychiatric disturbance following childbirth.

RESULTS

No significant differences between genotype or allelic frequencies were found between the two groups for either polymorphism.

CONCLUSION

The results indicate that variation at two common polymorphisms of the 5HT2A gene does not appear to play a major role in the development of bipolar affective puerperal psychosis.

Positive association between panic disorder and polymorphism of the serotonin 2A receptor gene

Family and twin studies have shown that genetic factors play an important role in the etiology of panic disorder. However, linkage and association studies using DNA markers have yielded inconclusive results. Increased serotonin neurotransmission may cause or be related to panic disorder. Assuming that genes regulating the serotonin system are involved in the pathogenesis of panic disorder, the authors searched for a genetic association of panic disorder with the serotonin 1A (HTR1A), 2A (HTR2A), and 2C (HTR2C) receptor genes. HTR1A, HTR2A and HTR2C polymorphisms were detected by the polymerase chain reaction method with analysis of restriction fragment-length polymorphisms (PCR-RFLP). The subjects were 63 biologically unrelated patients with panic disorder and 100 biologically unrelated normal control subjects who were native Japanese living in the western area of Japan. HTR1A and HTR2C showed no significant association with panic disorder. However, the frequency of the MspI A2 allele of HTR2A was significantly higher in the patients than in the normal control subjects. The study showed a positive association between panic disorder and the HTR2A gene, suggesting that HTR2A plays an important role in the pathogenesis of panic disorder.

Association of 5-HT(2A) receptor gene polymorphism with major affective disorders: the case of a subgroup of bipolar disorder with low suicide risk

BACKGROUND

The implication of serotonin in suicide and affective disease explains why the 5-HT(2A) receptor gene has been proposed as a candidate gene in these disorders, although with conflicting results.

METHODS

We analysed the distribution of the 5-HT(2A)-1438A/G genetic polymorphism in 192 patients with major affective disorder (127 bipolar disorders and 65 unipolar disorders) compared to 142 healthy control subjects.

RESULTS

We found a higher frequency of the A allele in affected patients than in control subjects (p =.034), this difference being particularly striking for the subgroup of patients with type I bipolar disorder (p =.015). Patients with no personal and/or familial history of suicide attempts mainly accounted for the excess of the A allele in affected patients.

CONCLUSIONS

The association detected in this study suggests that the 5-HT(2A) receptor gene may play a role in the genetic susceptibility to bipolar disorder, through a specific subgroup of bipolar type I patients with lower risk of suicidal behavior.

The 5-HT(2A) receptor gene 102T/C polymorphism is associated with suicidal behavior in depressed patients

Several lines of evidence suggest that genetic factors constitute an important determinant of suicidal behavior. A significant association between the 5-HT(2A)-C allele and suicidality has recently been reported. The aim of this study was to investigate whether the proposed association between 5-HT(2A)-102T/C polymorphism and suicidality could be replicated in a larger and independent sample of Spanish patients with major depression. The 102T/C polymorphism of the 5-HT(2A) receptor gene was analyzed in 159 patients with major depression (DSM-IV criteria) and 164 unrelated and healthy controls using a case control design. All individuals were subjects of Spanish origin. Significant differences in allele (chi-square = 4.13, df = 1, P = 0.04) and genotype (chi-square = 6.19, df = 2, P = 0.04) distributions were found between non-suicide attempters and suicide attempters. Moreover, those patients carrying 5-HT(2A)-C allele had more than five times the risk for attempting suicide than noncarriers (OR = 5.50, 95% CI = 1.18-35.20, P = 0.01). Our results replicate the proposed association between 5HT(2A)-C allele and suicidality in major depression. Moreover, no overall associations are detected when patients with major depression and controls are compared for 102T/C frequencies, suggesting that the increased risk for suicidality conferred by 5-HT(2A)-C allele is primarily associated with suicidal behavior and not with the diagnosis of major depression itself.

Association studies of bipolar disorder

Association studies in outbred populations represent an important paradigm for investigation of complex traits, such as bipolar disorder, both to follow-up regions of interest from linkage studies (by systematic linkage disequilibrium mapping and positional candidate studies) and for pure functional studies. The advantages of the association method include its relative robustness to genetic heterogeneity and the ability to detect much smaller effect sizes than are detectable using feasible sample sizes in linkage studies. The candidate gene approach is potentially very powerful, particularly when used within the context of a VAPSE (variation affecting protein structure or expression) paradigm, but a major problem is that the efficiency in the choice of candidates is inevitably a function of the level of previous understanding of disease pathophysiology. To date, most candidate gene studies in bipolar disorder have focussed on the major neurotransmitter systems that are influenced by medication used in clinical management of the disorder. Early studies often used anonymous markers in the hope of detecting linkage disequilibrium but recently direct examination of polymorphisms of known or presumed functional relevance has become more usual. Most studies in the literature have been of the unrelated case-control design with samples rarely exceeding 200-300 subjects. No definitive findings have yet emerged although there have been some interesting preliminary findings including those with polymorphisms within the genes encoding catechol-o-methyl transferase (COMT), monoamine oxidase A (MAOA) and the serotonin transporter (hSERT; 5-HTT). In this article we critically review the current status of the literature within the context of the important methodological issues and limitations inherent in the use of association studies for genetic dissection of bipolar disorder. We conclude by examining likely future directions and developments in the field.

5-HT2A receptor gene promoter polymorphism -1438A/G and bipolar disorder

Genetic factors, such as the genes involved in the serotonin pathway, probably play an important role in the pathogenesis of bipolar disorder, and serotonin type 2A (5-HT2A) receptor gene promoter polymorphism -1438A/G has been reported. This study investigated the association between -1438A/G polymorphism of 5-HT2A receptor gene promoter and bipolar disorder in a Korean population. Using the polymerase chain reaction, -1438A/G polymorphism typed in 142 patients with bipolar disorder and in 148 normal control subjects. Differences in genotype distributions and allele frequencies of -1438A/G between patients with bipolar disorder and normal control subjects were tested for significance using the chi-squared test. There were significant differences in genotype distributions [chi2 = 9.697, degrees of freedom (df) = 2, P = 0.008] and allele frequencies (chi2 = 7.284, df = 1, P = 0.007) of -1438A/G between patients with bipolar disorder and normal control subjects. Although further studies are necessary, these results in a Korean population suggest that -1438A/G polymorphism of 5-HT2A receptor gene promoter may be causally related to the development of bipolar disorder.

Variability in the 5-HT(2A) receptor gene is associated with seasonal pattern in major depression

The 102-T/C polymorphism of the 5-HT(2A) receptor gene was analysed in 159 patients with major depression and 164 unrelated and healthy controls using a case-control design. Allele and genotype frequencies did not differ between cases and controls. No differences according to sex, age of onset, melancholia, suicidal behaviour or family history of psychiatric illness were found. However, genotype distributions significantly differed between patients with seasonal pattern in their episodes (MDS) and patients with no seasonal pattern (N-MDS) (chi(2) = 10.63; P = 0.004). A seasonal pattern was 7.57 times more frequent in 102C-allele carriers than in 102T homozygous (95.1% of patients MDS carried 102C-allele vs 72% of patients N-MDS (chi(2) = 9.45, df=1, P = 0.002; OR = 7.57 (95% CI: 1.65--48.08)). These results suggest that variation in the 5-HT2A receptor gene may play a role in the development of major depression with seasonal pattern and support the existence of a genetic and etiological heterogeneity underlying the diagnosis of major depression.

5-HT2a receptor polymorphism gene in bipolar disorder and harm avoidance personality trait

The purpose [corrected] of this study was to investigate the relationship between bipolar disorder and the harm avoidance personality trait (HA), and the genetic contribution of the polymorphic DNA variation T102C in exon 1 of 5-HTR2a (chromosome 13q14-21) in bipolar disorder and HA personality trait. Forty bipolar patients and 89 normal subjects completed the TPQ questionnaire and were genotyped for 5-HT2a. Bipolar patients scored higher than normal subjects on the HA dimension. However, no contribution of the 5-HTR2a polymorphism on the bipolar disorder or on the HA personality trait emerged. Despite the limited sample size, these results exclude a major effect of the 5-HTR2a polymorphism on bipolar disorder and HA personality trait but not a minor effect.

Negative association between T102C polymorphism at the 5-HT2A receptor gene and bipolar affective disorders in Singaporean Chinese

BACKGROUND

Serotonergic system abnormalities have been implicated in the pathogenesis of bipolar affective disorders. The 5-hydroxytryptamine type 2A (5HTR2A) receptor gene located on chromosome 13 (13q14-21) can be considered as a candidate gene for bipolar affective disorder (BPAD).

METHODS

Seventy-two patients with BPAD and 74 normal population controls were genotyped with restriction fragment length polymorphism (RFLP) in the 5HTR2A receptor gene.

RESULTS

We report a negative association between 5HTR2A receptor gene and BPAD. The association was examined using a case-control design. Allele and genotype frequencies as well as homozygote-heterozygote distribution at the 5HTR2A receptor gene polymorphism were compared between the two groups. There were no significant differences in the allelic or genotype frequencies and the homozygote-heterozygote distributions.

LIMITATIONS

Patients were recruited from one hospital in Singapore. The case-control study design needs replication.

CONCLUSION

Our finding indicates that the 5HTR2A receptor gene polymorphism is not a major factor in the genetic susceptibility to BPAD in Singaporean Chinese.

Searching high and low: a review of the genetics of bipolar disorder

OBJECTIVES

To review the methodologies and findings in the genetics of bipolar disorder (BPD), and to suggest future directions for research.

METHODS

Reports of family, twin, adoption, linkage, association, cytogenetic, and animal model studies, and segregation analyses in English, were identified from multiple MEDLINE searches. Hand searches were carried out in bibliographies from review articles.

RESULTS

Family, twin, and adoption studies have provided strong evidence for a genetic etiology in BPD. Early reports of linkage of BPD to DNA markers at several chromosomal sites have not proven robust, perhaps because of the complex nature of BPD inheritance. However, linkage findings in the 1990s, on chromosomes 18, 21q, 12q, and 4p, have provided leads that are being pursued through both genetic and physical mapping. No gene has yet been definitively implicated in BPD.

CONCLUSIONS

Strategies for increasing the power to detect BPD genes include: (1) dividing the phenotype into genetically meaningful subtypes to decrease heterogeneity: and (2) ascertaining a very large family sample--a multicenter study now in progress will collect 700 bipolar I sibling pairs. BPD may result from several genes acting in concert so that new multilocus statistical methods could enhance the capacity to detect loci involved. Family-based association studies using a very large number of newly identified single nucleotide polymorphisms (SNPs) may allow for more efficient screening of the genome. As the Human Genome Project approaches its goal of isolating all genes by 2003, the data generated is likely to speed identification of candidate BPD genes.

An unstable trinucleotide-repeat region on chromosome 13 implicated in spinocerebellar ataxia: a common expansion locus

Larger CAG/CTG trinucleotide-repeat tracts in individuals affected with schizophrenia (SCZ) and bipolar affective disorder (BPAD) in comparison with control individuals have previously been reported, implying a possible etiological role for trinucleotide repeats in these diseases. Two unstable CAG/CTG repeats, SEF2-1B and ERDA1, have recently been cloned, and studies indicate that the majority of individuals with large repeats as detected by repeat-expansion detection (RED) have large repeat alleles at these loci. These repeats do not show association of large alleles with either BPAD or SCZ. Using RED, we have identified a BPAD individual with a very large CAG/CTG repeat that is not due to expansion at SEF2-1B or ERDA1. From this individual's DNA, we have cloned a highly polymorphic trinucleotide repeat consisting of (CTA)n (CTG)n, which is very long ( approximately 1,800 bp) in this patient. The repeat region localizes to chromosome 13q21, within 1.2 cM of fragile site FRA13C. Repeat alleles in our sample were unstable in 13 (5.6%) of 231 meioses. Large alleles (>100 repeats) were observed in 14 (1. 25%) of 1,120 patients with psychosis, borderline personality disorder, or juvenile-onset depression and in 5 (.7%) of 710 healthy controls. Very large alleles were also detected for Centre d'Etude Polymorphisme Humaine (CEPH) reference family 1334. This triplet expansion has recently been reported to be the cause of spinocerebellar ataxia type 8 (SCA8); however, none of our large alleles above the disease threshold occurred in individuals either affected by SCA or with known family history of SCA. The high frequency of large alleles at this locus is inconsistent with the much rarer occurrence of SCA8. Thus, it seems unlikely that expansion alone causes SCA8; other genetic mechanisms may be necessary to explain SCA8 etiology.

Serotonin-2A receptor polymorphism (102T/C) in mood disorders

Serotonergic dysfunction has been implicated in mood disorders and suicidal behaviors. This study examined the association between a serotonin-2A (5HT2A) receptor gene polymorphism (102T/C) and mood disorders. The genotype and allele frequencies did not differ between patients with mood disorders and control subjects. Furthermore, the 102T/C polymorphism was not found to be associated with suicidal history in mood disorder patients. Our results suggest that this polymorphism is unlikely to play a role in the genetic susceptibility to mood disorders.

Serotonin receptor variants in disease: new therapeutic opportunities?

This review describes a novel genetic approach to the assessment of receptor function that is based on association studies of polymorphisms within human genes. The realization that variations within human genes may significantly affect gene function has led to increased use of this approach in recent years. Analysis of polymorphisms within the human 5-HT2A receptor is used as a specific example of the application of association genetics to elucidate gene function. The interaction of many neuroleptics and antidepressants with 5-HT2A receptors points up the potential importance of this receptor for understanding and treating neuropsychiatric disorders such as schizophrenia and depression.

Evidence for an allelic association between bipolar disorder and a Na+, K+ adenosine triphosphatase alpha subunit gene (ATP1A3)

BACKGROUND

Disturbances in central nervous system Na+, K+ adenosine triphosphatase (ATPase) activity have previously been proposed as being involved in the pathophysiology of bipolar mood disorder.

METHODS

We have examined one particular alpha subunit of this enzyme for allelic association in a sample of 85 Irish bipolar patients and 85 matched controls.

RESULTS

There was evidence for an overall allelic association between the disease and a dinucleotide polymorphism within the ATP1A3 gene (p = .022). Subjects were then analyzed on the basis of a number of criteria, and the significance of the association increased when cases were divided based on the nature of the first episode. Patients who presented with a depressive episode first showed a significant association (p = .001) with this polymorphism.

CONCLUSIONS

The results presented here provide preliminary evidence of an association between bipolar disorder and an alpha subunit of Na+, K+ ATPase, the expression of which predominates in the brain.

Serotonin transporter gene and risk for bipolar affective disorder: an association study in Spanish population

BACKGROUND

The serotonin transporter (5-HTT) is an important candidate gene for the genetic transmission of manic depressive illness. Many studies of patients with affective disorders have found abnormalities in serotonin metabolism and dysregulation of the transporter itself. In the present study, we hypothesize that genetic variation in the 5-HTT gene (17q11.1-17q12) may have an effect in the etiology of manic depression.

METHODS

To test this hypothesis, we analyzed allele, genotype, and haplotype frequencies of two polymorphisms recently described in the 5-HTT gene (a variable number of tandem repeats in intron 2 and a deletion/insertion polymorphism in the transcriptional control region) in a sample of 88 patients with manic-depressive illness and 113 controls. Cases and controls were matched for ethnic and geographic origin.

RESULTS

No associations were found between any of these polymorphisms, tested individually or as haplotypes, and manic depression. Moreover, the genetic analysis by sex, presence/absence of psychiatric family history, and age of onset did not reveal significant differences in allele or genotype distributions.

CONCLUSIONS

Our results suggest that the genetic variability of the 5-HTT gene is not a major risk factor for manic depression.

No association between bipolar affective disorder and a serotonin receptor (5-HT2A) polymorphism

The serotonergic system is implicated in the pathogenesis of affective disorders. In particular, the role of the postsynaptic 5-hydroxytryptamine (serotonin) type 2 receptor (5-HT2) has been documented by several studies. The 5-HT2A receptor gene located on chromosome 13 (13q14-21) can be considered a candidate gene for bipolar affective disorder (BPAD). We tested association between a 5-HT2A receptor DNA variant and BPAD using a case-control design. Eighty-three BPAD patients and 129 unrelated normal controls, carefully matched for sex and geographical origin, were studied. Allele and genotype frequencies as well as homo-heterozygote distribution at the 5-HT2A receptor polymorphism were compared between the two groups. No significant allelic or genotypic associations were observed. There was no significant difference for homo-heterozygote distribution between the two groups. These preliminary results may indicate that in our sample the 5-HT2 receptor polymorphism studied is unlikely to play a role in the genetic susceptibility to BPAD.

Bipolar disorder: from families to genes

BACKGROUND

Genetic factors are known to contribute to the etiology of bipolar illness, but the actual genetic mechanisms remain to be clarified.

METHODS

This paper reviews the research undertaken to establish the genetic basis of bipolar illness and to elucidate the nature of its genetic predisposition.

RESULTS

The presented findings suggest that bipolar affective disorder is a heterogeneous condition characterized by a complex relationship between the genetic susceptibility and the clinical presentation. Linkage studies have generated promising and replicated findings on chromosomes 18 and 21.

CONCLUSION

In spite of the methodological difficulties inherent in the genetic study of psychiatric disorders recent investigations have made important advances and promise to identify specific susceptibility genes.

Serotonin2A receptor gene polymorphism in mood disorders

Genes that regulate the serotonin (5-HT) system including 5-HT receptors may be involved in mood disorders. We studied 5-HT2A receptor exons and the adjacent intron regions in 102 patients with mood disorders (71 depressive disorders and 31 bipolar disorders). In 34 mood disorder cases, the gene encoding the 5-HT1A receptor had been sequenced, but no disease-specific polymorphism was found. The substitution of C for T at position 102 in exon 1, which had been reported by Warren et al., was confirmed. The corresponding amino acid, serine, did not change. The allele frequency of C [corrected] at position 102 was significantly higher in patients with depressive disorders than in those with bipolar disorders and healthy control subjects. Furthermore, the mean age of onset in the patients heterozyous for the T and C alleles was lower than that in those homozygous for the C allele. No other polymorphism in the gene was found.

5-HT2A receptor and bipolar affective disorder: association studies in affected patients

The aim of this study was to investigate the possible involvement of genetic variation in serotonin receptors in the aetiology of bipolar affective disorder. The 5-HT2A receptor gene was systematically screened for genetic variants by single strand conformation polymorphism (SSCP) methods in subjects with bipolar affective disorder. Four polymorphisms (two structural changes, Thr25Asn and His4 M52Tyr, and two silent polymorphisms, 102-T/C and 516-C/T) which had previously been found in patients with schizophrenia and control subjects were detected. No novel polymorphisms were found in patients with bipolar affective disorder. These polymorphisms were genotyped in a sample of 129 patients and 252 controls of German origin and 176 patients and 182 controls of British origin. No strong associations were found between any of these polymorphisms and bipolar affective disorder. Genetic variation at the 5-HT2A receptor gene does not play a major role in the pathogenesis of the disorder.

Allelic association analysis of the 5-HT2C receptor gene in bipolar affective disorder

We have examined a structural variant of the 5-HT2C receptor (Cys23Ser) for allelic association with bipolar affective disorder in 88 cases and 113 controls. Overall, there was no significant difference in allele frequencies between the two groups, indicating that the 5-HT2C gene is not a major risk factor for bipolar affective disorder. However, when the subjects were analysed according to sex, there was a small excess of the serine ser23 allele in female cases (P = 0.04) and this effect was also seen if the ser23 allele was considered recessive (P = 0.03). A small increase in significance was found if only female cases with a known family history were included (P = 0.01). These results suggest that the ser23 allele may increase susceptibility to bipolar affective disorder in women.